A specialized heat-exchanger to further heat 165°C Steam to 500°C steam. Function otherwise like regular Heat-Exchangers. Use a less valuable resource to first heat water to 165°C seperately, ie. waste wood in regular burner-boilers, or solar-panel powered Electric-Boilers, thus saving the energy so used to heat 15°C water to 165°C steam.
My nuclear powerplant in UPS friendly: I took my 1.1 GW (2*4 reactors) smart-powerplant blueprint and made it into a single entity. Blueprint-String is in the .zip This entity costs all the ingredients the original construction would cost (unless I missed something; this is a WIP). It consumes 1 fuelcell every 25 sec: the original eats 8 every 200 sec under full load. It pauses consumption, if not enought energy is used up - as the original would in a way. (Currently not rotatable and the water-requirement at the offshore pumps is not checked either/doesnt work)